1. Field of the Invention
This invention provides a method for enhanced growth of carbon nanotubes (CNTs). The specific enhancement manifests itself as a substantial increase in the length of the CNTs under identical catalytic growth via chemical vapor deposition (CVD). The method includes an ultrasonication and exposure to water step which involves (1) immersing the catalyst deposited on the substrate in a bath containing a liquid, such as, for example, an aqueous solution or a non-aqueous solution, (2) subjecting the catalyst/bath system to acoustic waves generated from an ultrasonic apparatus (e.g., sonicator), and (3) exposing the catalyst to water (e.g., via rinsing). Following the ultrasonication and exposure to water step, the CNTs are grown utilizing CVD. Comparison of CNTs grown with the ultrasonication and exposure to water step, and CNTs grown in an identical manner but without the ultrasonication and exposure to water step, reveals a significant increase in the length of the CNTs when the ultrasonication and exposure to water step is included. The range of applications for CNTs with increased length covers gas diffusion layers for fuel cells to thin conductive films.
2. Description of Related Art
Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. Carbon nanotubes (CNTs) possess certain electronic and mechanical properties, making them candidates for applications relating to composite materials, nanoelectronics, sensors, and electron field emitters. CNTs can be utilized individually or as an ensemble to build a variety of devices. For instance, individual nanotubes have been used as tips for scanning probe microscopy and as mechanical nano-tweezers. Ensembles of nanotubes have been used for field emission based flat-panel displays, and it has been suggested that bulk quantities of nanotubes may be used as a high-capacity hydrogen storage media. The electronic behavior of CNTs is closely related to their structure (i.e., tip curvature, radius and composition, nanotube length, and chirality).
Carbon nanotubes have attracted much attention due to their unique physical properties. These properties range from efficient electron field emission to immense surface to volume ratio suitable for catalytic processes and high electrical conductivity suitable for thin film-based devices. For some applications such as field emission, exact control of the size, length and position of the nanotube may be critical. Other applications (e.g., thin films, catalysis, composite materials) require carbon nanotubes of good size uniformity and substantial length. One example of a method of generating carbon nanotubes involves catalytic growth via chemical vapor deposition (CVD). To this end, researchers have sought ways to increase the length of CVD-based nanotubes and nanotube bundles.
The length of the nanotube is limited by the deposition parameters as well as catalyst-poisoning phenomenon. One method has utilized introduction of water vapor along with the reactant gases during the chemical vapor deposition process, which resulted in growth of nanotubes with considerable length. However, the above-mentioned method is complicated by the requirements of very precise control of the water vapor pressure, and the costly additions to a conventional CVD apparatus necessary to properly control the introduction of water vapor during the CVD process.
Therefore, a need has arisen for an alternative simple method of generating long nanotubes which avoids the complications discussed above.